What did the high court decide today? Victims of deep vein thrombosis (DVT) reacted angrily today after a high court judge effectively blocked their case for damages against up to 30 airlines.

Mr Justice Nelson ruled that, under the terms of the 1929 Warsaw convention (that governs all international carriage of persons by aircraft for reward), DVT cannot be deemed an "accident" and therefore airlines cannot be held responsible for the condition. The Warsaw convention only allows for recovery of compensation in respect of personal injury or death caused by an accident.

However he did give leave for the 55 claimants, a group of DVT survivors and the families of people who have died from the condition, to appeal. Solicitors for the claimants and some MPs have argued that the convention is now out of date.

Counsel Stuart Cakebread told the court that the phenomenon did not strike "rarely and unexpectedly" and added: "We are dealing with repeated, statistically predictable and relatively frequent deaths and injuries inflicted, the claimants say, by the acts and neglect of those they are paying to look after them."

Hopes were also boosted this morning by news of a ruling in favour of a group of DVT claimants in Australia.

What did the Australian court say? The Victoria state supreme court in Australian today cleared the way for passengers who developed "economy class syndrome" during long-haul flights to sue airlines. Unlike the high court, the Australian judge rejected a claim by Qantas Airways and British Airways that a clot suffered by a Sydney man on board its flights should not be classed as an accident.

The Australian ruling means the case will now probably go to trial late next year. The test case, involving a 59-year-old man who no longer works after developing a blood clot while on a three-day return business trip from Sydney to London, is the first of 497 potential lawsuits regarding deep vein thrombosis, or DVT, which have been lodged with an Australian law firm.

What were the British claimants hoping to achieve?If their appeal is successful, the British group vows to take action against up to 30 airlines, including British Airways, Delta Airlines and American Airlines. As of January 2002, 286 people had come forward and contacted solicitors about potential DVT claims against airlines. If the airlines are forced to defend the group action, it could cost them millions in compensation.

What have the airlines said about DVT? A spokesman for British Airways has said that the company would resist claims against it in the context of advice given by the government and the World Health Organisation that no specific link between flying and DVT had been established.

What is deep vein thrombosis? DVT stems from the formation of blood clots in the calf veins. The condition can be deadly if these clots break free, travel to the lungs and block the flow of blood.

What causes it? Being stationary in a sitting position for hours on end. Doctors are not sure if the condition is made worse by air travel than any other occupation such as desk work, train or bus travel, where people remain seated for hours on end. What seems clear is that being crammed into airline seats on long-haul flights can cause DVT, also known as "economy class syndrome".

How common is it? Government medical experts say it leads to around 10 deaths a year. But Australian lawyers acting on behalf of passengers who claim to have suffered from DVT reckon it causes as many as 60 deaths a year here. In Australia, 2,700 passengers are seeking damages from airlines after nurturing clots.

Has anybody proved the causes and how can it be prevented? A University College London research team got 200 people to take a long haul flight. 84 of them were given special socks called compression stockings to wear. Not one of them developed deep vein thrombosis. One in ten of those who didn't wear the stockings did develop the condition and were immediately treated for it.

Who is most susceptible to DVT? Those thought to be most at risk include people aged over 40, heart patients, people who have had a clot before, those with relatives who have suffered one, people who have had recent surgery, who have had a stroke or cancer and women who are pregnant, on the Pill or on hormone replacement therapy.

What are the government and airlines doing about it? Some airlines are offering compression socks on long haul flights. The government has issued a set of guidelines to airline passengers. It advises passengers on long-haul flights to perform leg exercises in their seats and to get up and walk around when possible. The government also advises people with conditions that make them more vulnerable to clots to seek medical advice before taking a long trip.